1. Field of the Invention
The invention relates generally to digital amplifiers, and more particularly to systems and methods for improving the stability of feedback and/or feed-forward subsystems in digital amplifiers.
2. Related Art
In an all digital pulse width modulation (PWM) amplifier system a digital input signal is processed by digital components to produce a PWM signal, which is then used to drive an output stage. The output stage produces an analog signal that is low-pass filtered and used to drive a load such as a loudspeaker.
Digital PWM amplifier systems often include feedback to reduce noise and distortion in the output. The system may also include feed-forward correction, which is based on information other than the amplifier's output, to reduce noise and distortion. In a digital PWM amplifier with digital feedback (using ADCs), part of the feedback loop and feed-forward path, specifically the high voltage output stage and the analog portion of the loop/path, including the analog-to-digital converter (ADC), will be outside the digital PWM device. Because the external loop/path is outside the digital PWM device, it is not always guaranteed to be operating within the parameters for which the PWM device was designed. This may be a result of design errors, user errors, damaged components, etc.
In conventional amplifiers, the entire amplification system is integrated, so the design of the entire feedback loop is known, and the feedback loop can simply be designed to be stable. When the digital portion of the amplifier is designed independently of the analog portions of the amplifier, such as output stage and analog portions of the feedback path, the design of these analog components is not known, and the digital design cannot assume any particular specifications or behavior for the analog components. It is instead necessary for the digital design to be capable of handling any possible behavior by the analog components in order to maintain the stability of the feedback system.
It would therefore be desirable to build feedback/feed-forward (FB/FF) protection into the system, aside from any over-current, shoot-through, and over-temperature protection mechanisms that may be incorporated into the design, in order to counter misbehaved feedback and feed-forward. It would further be desirable for the FB/FF protection to provide not only protection from errors or failures in the FB/FF, but also a graceful transition between normal operation and a disruptive condition. One of the primary goals of such FB/FF protection would be to maintain the stability of the PWM system and to keep audio amplifier operational as long as possible under non-ideal conditions. A secondary goal would be to help prevent damage to the amplifier and the speaker by working in parallel with the other protection mechanisms.